An analysis of the infrared and Raman spectra of the formic acid dimer (HCOOH)2

Abstract

Ab initio molecular quantum mechanical methods have been used to study the formic acid dimer and several isotopomers. Structures and harmonic vibrational frequencies have been predicted for the monomer, the equilibrium dimer, and the transition state for simultaneous two-hydrogen transfer. In an attempt to systematize results obtained at various levels of theory, three different basis sets were used: minimum, double-zeta (DZ), and double-zeta plus polarization (DZ+P). The predicted self-consistent-field (SCF) dissociation energies D/sub e/ are 15.1 (minimum basis), 19.3 (DZ), and 14.3 (DZ+P) kcal/mol. Further consideration of zero-point vibrational energies and finite temperature effects allows a comparison with several experimental values for ..delta..H/sub 300/. The infrared and Raman intensities (as well as the vibrational frequencies themselves) are compared with available experimental results. Where comparison is possible, the agreement is generally good. An exception occurs for v/sub 18/, the O-D stretch of the (HCOOD)/sub 2/ molecule, where it is concluded that the experimental analysis may be incorrect.